Duke, D., Swantek, A., Kastengren, A., Fezzaa, K. et al., "Recent Developments in X-ray Diagnostics for Cavitation," SAE Int. J. Fuels Lubr. 8(1):135-146, 2015, doi:10.4271/2015-01-0918.
Cavitation plays an important role in fuel injection systems. It alters the nozzle's internal flow structure and discharge coefficient, and also contributes to injector wear. Quantitatively measuring and mapping the cavitation vapor distribution in a fuel injector is difficult, as cavitation occurs on very short time and length scales. Optical measurements of transparent model nozzles can indicate the morphology of large-scale cavitation, but are generally limited by the substantial amount of scattering that occurs between vapor and liquid phases. These limitations can be overcome with x-ray diagnostics, as x-rays refract, scatter and absorb much more weakly from phase interfaces. Here, we present an overview of some recent developments in quantitative x-ray diagnostics for cavitating flows. Measurements were conducted at the Advanced Photon Source at Argonne National Laboratory, using a submerged plastic test nozzle. X-ray radiography provides quantitative line-of-sight density measurements of cavitation void fraction by measuring relative changes in absorption, with a relatively constant uncertainty of 2% of a typical peak value. Single point measurements are built up into a vapor fraction distribution by raster-scanning the nozzle through the fixed beam. X-ray fluorescence, a novel alternative diagnostic, can provide a similar quantitative point measurement, but measures the emission of fluorescent x-rays from an excited tracer in the fuel rather than the directly transmitted beam. An uncertainty of 1.1% of the projected void fraction is achieved, giving a more precise measurement near the nozzle wall. X-ray phase contrast imaging provides a temporally and spatially resolved view of the flow due to absorption and diffraction, revealing small-scale dynamic behaviors that are difficult to observe in point-based measurements.